Selenium is a chemical element whose concentration in fluid and vapor streams is governed by enforceable regulations in both drinking water and industrial discharges. It is often present in aqueous streams along with other contaminants, such as mercury, fluoride and arsenic. Selenium salts are toxic in large amounts to humans, fish and animals. Selenium occurs at approximately 0.7 milligrams per liter in the earth's crust and concentrates in plants, sulfur deposits, sulfide minerals of copper and molybdenum and fossil fuels. The contaminant is prevalent in many waste streams including those that result from copper mining, coal mine drainage, coal-fired power plants, agricultural runoff and petroleum production and refining. It may also be present from glassmaking, pigments, and electronics manufacturing. Selenium is under increasing government regulation and worldwide corporate scrutiny. The removal of selenium from aqueous fluids to levels as low as 5 parts per billion is a focus of the mining, agriculture, power generation and oil and gas industry sectors and much attention is given to proper handling and disposal of waste materials classified as hazardous or toxic. Primary sources are the selenium impurities in metal sulfide ores, where it partially replaces the sulfur.
The chemistry of selenium is a polyatomic nonmetal sometimes considered a metalloid that rarely occurs in its elemental state in nature or as pure ore compounds. In water, selenium generally is present as an oxyanion or as an organic compound. There are two primary oxyanion species, selenite and selenate.
Selenium, especially the selenate oxyanion, is not effectively removed by many processes, substances or methods. Typical adsorbent medias such as activated carbon, organoclays or plain activated alumina do not remove significant levels of selenium and are ineffective in achieving emerging targets of 5 parts per billion in water streams. Functionalized activated alumina has been used as an absorbent of selenium compounds. Occasionally in streams where selenium is present as selenate and in the presence of certain anions and levels of ionic strength, the selenium removal by functionalized alumina is not well-sustained. This represents a problematic and often prohibitive loss in media bed life and capacity.